Wraith
Bettyswollocks
Or do that, I did something similar in secondary school but with BIOS, setting a password you had to enter before the system would boot.
I wanted to do something on the library PC, I think it was a 486 DX 2 66mhz at the time but the kids on it before me wouldn't get off it so when I finally went on there and did what I needed to do. I put a password on it.
Rather than asking me what the password was, they paid someone to remove it.
The other one to do is print screen of the desktop, set as wallpaper and hide the icons and start bar.
